I'm not sure about buying premade buttercream, I always make mine. But I do use chocolate and then dye it black. It's so much easier than starting with white frosting. I've never had any trouble with bleeding, just make sure you don't thin the black frosting out too thin before you pipe it. HTH!

Yes, as trebakes said, get a tube of Wilton colored icing. The color is soooooo saturated you can easily remove all the icing from the tube and add at least an equal amount (probably more) and still have a good color. I usually use ti for red or black. If the taste is that bad add a bit of vanilla.

To remove the icing out of the tube, cut off the bottom of the tube and scrape it out; mix with your icing and proceed as you would using any piping bag.